Emily is 3 times as old as Christopher and is also 10 years older than Christopher. How old is Christopher?
Solution: We can use the given information to write down two equations that describe the ages of Emily and Christopher. Let Emily's current age be $e$ and Christopher's current age be $c$ $e = 3c$ $e = c + 10$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $c$ , and both of our equations have $e$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 =$ $3c$ $-$ $ (c + 10)$ which combines the information about $c$ from both of our original equations. Solving for $c$ , we get: $2 c = 10$ $c = 5$.